The two most important active cooking chemicals used to treat comminuted cellulosic fibrous material during the sulfate or "kraft" cooking process are sodium sulfide, Na.sub.2 S, and sodium hydroxide, NaOH. An expression used in the pulping industry to designate the amount of cooking chemical present during pulping is "effective alkali". Since sodium sulfide hydrolyzes in an aqueous medium to form sodium hydroxide, or "alkali", and sodium hydrosulfide, "effective alkali" is defined as the total of the concentration of sodium hydroxide plus one-half the concentration of sodium sulfide (expressed as NaOH), or EQU effective alkali=[NaOH]+1/2[Na.sub.2 S]
Various methods of treating cellulose material with alkali have been presented. For example, in conventional kraft cooking, all the cooking chemicals, or effective alkali, are introduced at the beginning of the cooking process, that is before the impregnation phase. In this process the alkali is gradually consumed as the treatment progresses. Typically, the effective alkali in the cooking liquor for a conventional kraft cook is greatest at the beginning of the cook and the least at the end of the cook.
Following studies performed at the Swedish Forest Products Research Institute, STFI, in the early 1980s, so-called "modified cooking" was introduced in the early-to-mid 1980s. As described by Sjoblom, et al. [Paper and Timber, 1983, No.4, page 227], one of the goals of this type of cooking was to provide a "low and uniform concentration of effective alkali" as a means to improve both viscosity and yield. The variation or profile of the concentration of effective alkali throughout a modified continuous kraft cook was illustrated and contrasted with the variation for conventional kraft cook by Johansson, et al. in 1984 [Svensk Papperstidning, No. 10]. Johansson, et al. described how such modified cooking with more uniform alkali concentrations produced improved pulp viscosity, better bleachability and lower environmental load, among other things. This work and subsequent trials by others established that a low and uniform effective alkali distribution in both continuous and batch kraft cooking was the preferred mode of operation. This low and uniform treatment of pulp became the cornerstone of the MCC.RTM. and EMCC.RTM. digesters and cooking processes, as sold by Ahlstrom Kamyr of Glens Falls, N.Y., which became very popular in the industry in the late 1980s and early 1990s.
Though the benefits of a low and uniform effective alkali on pulp viscosity and yield have been accepted in the industry, it has been surprisingly discovered that such profiles do not produce pulp having the highest intrinsic fiber strength.
The term "pulp viscosity" had been associated directly with pulp fiber strength. A pulp's viscosity is sometimes interpreted as an indirect measure of the a pulp's relative fiber damage, or depolymerization. The lower the viscosity, according to this reasoning, the more the fiber is damaged. It has often been assumed that a damaged fiber produced a weaker pulp and hence reduced viscosity was interpreted as reduced intrinsic fiber strength. However, it has been found according to the present invention that high effective alkali concentrations and/or high pH during bulk and residual delignification, though possibly reducing a pulp's viscosity, produce a pulp having higher intrinsic fiber strength. For instance, laboratory cooks using high alkali concentrations during cooking, for example with effective alkali concentrations of 32 g/l required only about 40% of the H-factor as is conventional to produce a kappa number of 21 for softwood and increased pulp strength. Also, cooking in high effective alkali and/or pH gives pulp with better bleachability (as recognized in the literature). Furthermore, the high effective alkali concentration yields a high residual alkali concentration in the spent cooking liquor which can be effectively used to pretreat chips prior to the bulk delignification stage of cooking. The present invention permits the use of lower cooking temperatures and/or shorter cooking times to effect cooks comparable to conventional methods. In other words, by using this invention cooking vessels can be designed smaller and cheaper. This also means that existing cooking vessels, which are limited due to the existing processes, can be made to produce more pulp per unit time without increasing their temperature or effective alkali charge.
The terms "bulk" and "residual", as applied to delignification, are standard concepts in the pulp and paper art, and are defined in "Pulp and Paper Manufacturing", Volume 5, Alkaline Pulping, Grace et al, Technical Section, Canadian Pulp & Paper Association, 1989, pages 60-62. In brief, "bulk delignification" is that phase of delignification during which most of the lignin is removed with a selectivity that is high compared to that during the initial phase that it follows, while "residual delignification" is a phase after bulk delignification characterized by a much slower delignification rate, increased yield loss, and increased alkali consumption per unit of lignin removed.
According to the present invention the exact mechanism that achieves desired results is not completely understood. High pH--which is not identical to high alkali (and may be a more accurate indicator of active cooking chemicals), although high alkali normally creates a condition of high pH--may be more significant than high alkali itself, and the combination of the two may be most significant.
The broadest aspect of this invention comprises a method of cooking comminuted cellulosic fibrous material employing high effective alkali concentrations in at least one stage of treatment. This high alkali concentration is preferably practiced in the bulk delignification stage of cooking. Preferably, this effective alkali concentration exceeds 15 g/l (more preferably 20 g/l) during both bulk and residual delignification. The method of the invention may be performed continuously or in batch mode, in a single-vessel or multiple-vessel system, in a hydraulic or vapor/liquor-phase digester; in the preferred embodiment, however, the method of the invention is performed continuously, using conventional continuous digesters of a variety of types.
According to one aspect of the present invention, a method of treating comminuted cellulosic fibrous material to produce cellulose chemical pulp with enhanced intrinsic fiber strength compared to pulp produced by convention or modified cooking methods, is provided. The method comprises the steps of continuously and sequentially: (a) Treating (e.g. impregnating) the comminuted cellulosic fibrous material with a first cooking liquor having a first effective alkali concentration which is greater than 10 g/l. (b) Further treating the (e.g. now impregnated) material with the first cooking liquor so as to consume alkali from the first cooking liquor, so that the effective alkali concentration of the spent first liquor is reduced to about 10 g/l or less. (c) Extracting the spent first cooking liquor from the material. (d) Treating (e.g. impregnating) the material with a second cooking liquor having a second effective alkali concentration greater than 25 g/l and greater than the first concentration, and a pH of at least 13, the second cooking liquor providing at least 50% of the total alkali to be consumed by the material in the production of chemical pulp. (e) Cooking the material with the second cooking liquor at cooking temperature to produce chemical pulp and a spent second cooking liquor having an effective alkali concentration of greater than about 15 g/l (e.g. greater than about 20 g/l). And (f) extracting the spent second cooking liquor from the pulp.
Step (b) is preferably practiced using as the first cooking liquor the spent second cooking liquor from step (f); some additional cooking liquor (typically white liquor) may be added. In typical existing prior art pulping systems, "conventional" or "modified," the cellulose material must be treated with cooking chemicals to pretreat or impregnate the material prior to formal cooking or bulk delignification. Normally, more than about 25%, sometimes more than 50%, of the total alkali consumed in the production of pulp is consumed during this pretreatment, that is, in step (b). However, among other advantages, the present invention obviates the need to introduce fresh cooking chemical in this pretreatment stage by using the residual alkali present in the black liquor produced in a high-alkali cooking process as the source of cooking chemical in this stage.
The second cooking liquor preferably is white liquor combined with wash liquor, or black liquor, and desirably more than about 80% of the total amount of white liquor (total alkali to be consumed) to be used to produce the pulp should be added in step (d) as the second cooking liquor. Wash liquor is used to dilute the white liquor for the second cooking liquor to provide a desired effective alkali concentration, and a favorable liquor to wood ratio. The practice of step (d) may also inherently result in the heating of the material to cooking temperature, or heating may be practiced separately, cooking temperature typically being in the range of 140.degree.-180.degree. C., typically between 150.degree. and 170.degree. C. Preferably, the liquor present in the digester as the second cooking liquor has an effective alkali of greater than about 25 g/l, e.g. about 25-60 g/l, typically about 30-50 g/l. These ranges of effective alkali concentration are typically provided by diluting the fresh cooking chemically, initially at about 90 g/l or more effective alkali, with any available source of dilution. This dilution may include black liquor, wash filtrate or cold blow filtrate, among others. The invention also includes the subsequent steps of cooling and washing the pulp, and prior to step (a) the material is preferably steamed to heat it and remove air from it. Also, steps (a), (b), (d) and (e) may be practiced either co-currently or counter-currently (flow of material to the flow of cooking liquor). The spent liquors extracted in steps (c) and (f) should be kept separately, and used for different purposes, typically the liquor from step (f) being used to preheat the second cooking liquor, and then flashed, with the remaining liquor used as the first cooking liquor while the steam is fed to the chips bin or presteaming vessel for pretreatment of the material, and the liquor from step (c) passed to conventional recovery in a kraft mill.
The invention also relates to a method of producing chemical pulp having enhanced intrinsic fiber strength from comminuted cellulosic fibrous material, comprising the steps of continuously and sequentially: (a) Treating (e.g. impregnating) the comminuted cellulosic fibrous material with a first cooking liquor having a first pH which is more than about 13.0 (e.g. more than about 13.2). (b) Further treating the (impregnated) material with the first cooking liquor so as to consume alkali from the first cooking liquor, so that the residual pH of the first cooking liquor is about 13.0 or less (or about 13.2 or less). (c) Extracting the spent first cooking liquor from the material. (d) Treating (e.g. impregnating0 the material with a second cooking liquor having a second pH of about 13.5 or greater (e.g. about 13.7 or greater) and greater than the first pH, the second cooking liquor providing at least 50% of the total alkali to be consumed by the material in the production of chemical pulp. (e) Cooking the material with the second cooking liquor at cooking temperature to produce chemical pulp and a spent second cooking liquor having a residual pH of at least about 13.0 (e.g. about 13.4 or greater or about 13.6 or greater); and (f) extracting the spent second cooking liquor from the pulp.
The invention also relates to a kraft pulp with enhanced intrinsic fiber strength and bleachability compared to kraft pulp produced by conventional and modified cooking. The kraft pulp according to the invention is produced using one or both of the methods as described above.
It is the primary object of the present invention to provide enhanced intrinsic fiber strength and enhanced bleachability chemical pulp by cooking with high alkali concentration and/or pH. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.